1. Field of the Invention
The present invention relates in general to a quality inspection method for inspecting printed matter, and more particularly to a method for setting a reference value to be used in the detection of white damage and/or black damage caused by an insufficiency and/or surplus of fed ink upon the commencement of printing and to such a method for detecting the white damage and/or the black damage.
Also the present invention relates to a method for inspecting print depth variations, that is applicable in the detection of a defective printing of a pattern such as figures, characters and the like on printed matter and in the detection of various contaminations of printed matter.
2. Description of the Prior Art
Heretofore, a method of inspecting printed matter which relied upon human visual sense on an off-line basis was principally employed. This is due to the facts that printed matter has different print patterns at various locations thereon, and that it has been believed that the inspection of printed matter requires the detection of slight differences that can only be determined by human visual sense. On the other hand, in response to the desirability to evaluate printed matter during printing, judgements were made during printing after converting an image on printed matter into a stationary image by effecting stroboscopic illumination synchronized with the printing speed or by employing a rotary mirror rotating at a high speed synchronously with the printing speed. However, even these measure cannot be considered automated in view of the fact that such inspection methods also rely upon human visual sense. In addition, another inspection method has been carried out in which a color patch is printed simultaneously with the printing of the pattern of printed matter and an inspection of the printed matter is simulated by inspecting the color patch. However, according to this method, in the event that printing faults (oil dropping, contamination, etc.) should arise in the pattern, they would be overlooked, and so, it cannot be said that such a method will function as effectively as an automated inspection machine.
On the other hand, in the "printed matter inspection apparatus" proposed recently in Laid-Open Japanese Patent Specification No. 56-98634 (1981) or Laid-Open Japanese Patent Specification No. 59-109832 (1984), a system for inspecting printed matter on an in-line basis by making use of a line sensor is disclosed. This system will be outlined with reference to FIG. 8.
A web-like printing paper sheet 3 fed from a roll of paper 2 mounted in a feeder section of a printing machine is, after being printed in a printing section 1, conveyed to a dryer and a folding machine (not shown). In the "printed matter inspection apparatus", in order to inspect a printed condition after printing commences, pattern information detected by a line sensor in a detecting section 4 is input to a processor circuit 6 while the timing for sampling is controlled by a rotary encoder 5 mounted to the printing section, and the pattern information is judged. As a result, if the printed condition has been judged to be abnormal, the apparatus can respond by issuing an alarm, marking the abnormal portion, rejecting the printed matter and the like.
According to the above-described system, since an entire patterned area of printed matter can be automatically inspected on an in-line basis, the previously described shortcomings are not present, and an advantageous inspection can be carried out.
The image information input through the detecting section 4 serves double roles as reference information and as inspection information within the processor circuit 6. The reference information is image information of a pattern on the printed matter at the time when an operator has judged that printing is normal, and this image information is stored within a reference information memory. On the other hand, the inspection information is information input from the detecting section 4 after the reference information has been stored in the memory, and which inspection information is compared with the above-described reference information. Whether the printed matter is acceptable or not is determined by judging whether the difference between the reference information and image information has exceeded a preset allowable limit, and thus the inspection of the printed matter is conducted on an in-line basis.
It will be understood that when employing the above-mentioned system, before the reference information is input to processor circuit 6, that is, during the period from the commencement of printing to the time normal printing occurs, inspection of the printed matter is impossible.
In practice, since unacceptable printed matter occurs in large amounts during the period from the commencement of printing to the time a normal printing state arises, especially in a rotary press in which there consequently occurs a large loss of printing paper, after the commencement of printing it is desired to automatically determine the exact point in time at which normal printing has begun. It is obvious that the printed matter inspecting apparatus according to the above-described prior invention is not effective at all in meeting this desire, and a solution to this problem is still sought.
In addition, according to the above-described system shown in FIG. 8, while it becomes possible to inspect an entire pattern on printed matter, the reference information and the inspection information to be processed constitute a large amount of information. Hence, the inspection apparatus in the prior art must have a complicated processing unit, is slow to carry out the inspection or carries out the inspection with insufficient accuracy.